In the art, zinc-plated steel sheets intended for use in the fields of automobiles, household electric appliances, building materials and the like are generally subjected to phosphate treatment or chromate treatment for the purpose of improving their corrosion resistance and coating adhesion, and the like. In particular, the process comprising chromate sealing treatment following zinc phosphate treatment is in wide use since it is highly effective in improving corrosion resistance and coating adhesion. However, with the recent background of the increasing interest in environmental problems, it is desired that a surface treatment technology without using any highly toxic chromate be developed. Thus, the following technologies have been proposed.
A zinc phosphate composite-treated steel sheet has been disclosed which has, as a first layer on the zinc-plated steel sheet surface, a zinc phosphate coating with a coating weight of 0.2 to 2.5 g/m2 and containing at least one metal selected from among nickel, manganese and magnesium and, thereon, as a second layer, an organic coating based on at least one organic resin selected from among ethylene resins, epoxy resins, urethane resins and acrylic resins (cf. e.g. Japanese Kokai Publication 2001-105528, page 2).
An organic composite-treated zinc-plated steel sheet has been disclosed which has a zinc plating coating, a zinc phosphate coating of not less than 0.3 g/m2 and an organic coating of 0.3 to 2 g/m2 as formed on the steel sheet surface in that order, wherein the zinc phosphate coating contains Mg, the Mg/P ratio (by weight) in the zinc phosphate coating is not less than 0.15 and the amount of Mg is not less than 20 mg/m2 (cf. e.g. Japanese Kokai Publication 2001-131763, page 2).
The zinc-plated steel sheets treated in such a manner are generally pressed for shaping and then subjected to alkaline degreasing and washing and are used either as such or after coating. In the automobile field, in particular, the car bodies in which the zinc-plated steel sheets treated in the manner mentioned above are used are conveyed on a coating line and are thus subjected successively to alkaline degreasing, zinc phosphate treatment, electrodeposition coating, intermediate coating, and top coating. Therefore, the coatings formed on the zinc-plated steel sheets are required to be firm and strong so that they may not be dissolved or deteriorated even upon contact with an alkaline degreasing solution and a zinc phosphate treatment solution.
However, when the zinc phosphate composite-treated steel sheets and organic composite-treated zinc-plated steel sheets obtained by such methods as mentioned above and the like methods are subjected to alkaline degreasing treatment after shaping thereof, the coatings are dissolved or deteriorated, so that poor corrosion resistance after the degreasing treatment and poor coating adhesion will result.
In view of the state of the art as discussed above, it is an object of the present invention to provide an inorganic-organic composite-treated zinc-plated steel sheet whose treated coatings do not contain any hazardous chromate and which is excellent in corrosion resistance after alkaline degreasing and coating adhesion.